Optical systems comprising high-bandwidth optical interconnects employ optical links to carry high-order waveforms from a transmitter to a receiver using optical fiber. Examples of optical systems that comprise high-bandwidth optical interconnects may include, but are not limited to, data centers, computer clusters, optical backplanes, metro dense wavelength-division multiplexing (DWDM), long-haul DWDM, passive optical networks (PONs), metro optical transport networks, long-haul optical transports, computer interconnects, and backhauls for wireless systems. Transmitters comprise a digitally-driven optical modulator that is configured to convert a multi-bit direct digital drive to an optical waveform. Examples of optical modulators include, but are not limited to, a multi-segment Mach-Zehnder interferometer modulator and a multi-segment electro-absorption modulator. An optical modulator comprises a plurality of phase shifter segments and is configured such that each phase shifter segment is driven with a digital bit stream to generate a desired optical waveform.
Existing optical modulators use a power-of-two length reduction relationship between the phase shifter segments where each subsequent phase shifter segment is half the length of the previous phase shifter segment from the most significant bit (MSB) to the least significant bit (LSB). For example, the MSB phase shifter segment is 128 times longer than the LSB phase shifter segment in an eight-bit optical amplitude modulator. The LSB is the bit position in a binary number with the lowest value. The MSB is the bit position in a binary number with the greatest value. Existing optical modulators are focused on the linearity of digital-to-analog conversions and the optimization of the driving voltage. These optical modulators apply the same driving voltage to all of the phase shifter segments.
Because the phase shifter segments are electrically driven, electrical crosstalk occurs between the phase shifters segments. Electrical crosstalk is the voltage induced on a victim phase shifter segment due to a change of voltage on an aggressor phase shifter segment. The electrical crosstalk acts on the victim phase shifter segment to produce a parasitic optical phase change. The length of the victim phase shifter segment determines the size of the parasitic optical phase change. In essence, the length of the victim phase shifter segment amplifies the electrical crosstalk from the aggressor phase shifter segment. As a result, the worst case is the electrical crosstalk from the LSB phase shifter segment to the MSB phase shifter segment, because the MSB phase shifter segment is very long. Electrical crosstalk reduces the number of bits that can be resolved, known as the effective number of bits (ENOB). As the baud rate of higher-order modulation increases, less significant bits are swamped by electrical crosstalk and the penalty due to electrical crosstalk in the transmitter becomes more important. It is desirable for a modulator to support high-order modulation while reducing the effects of electrical crosstalk.